This invention relates to a small particle detection system using a light scattering method in which small particles floating in the air are detected by measuring the scattered light therefrom.
The techniques for measuring the small particles contained in a sample air stream, by applying a beam of light to the sample air stream to detect the scattered light therefrom are generally known. In order to improve the detecting performance in such techniques, it is effective to improve the intensity of the beam of light in use. Accordingly, a laser oscillator is used as a light source instead of a lamp. These systems are disclosed in, for example, "O plus E", December 1982 issue, page 78. In order to improve the detecting performance in these systems, it is effective to improve the intensity of the beam of light in use. Therefore, a high output power laser device for increasing the level of the laser output, or a system for passing sample aerosol through the laser resonator is used. The former causes the dimensions of the detecting system and the level of input electric power to increase. In the latter, the diameter of a laser beam from the laser resonator is determined depending upon the laser oscillating conditions, and the diameter of the beam in the position of a detecting cell is not necessarily set to an optimum level for the measurement of the scattered light. In addition, the degree of freedom of arranging the detecting cell is restricted.
The latter system, i.e. the system in which a sample air stream is introduced into a laser resonator so as to be irradiated with a laser beam therein, for making effective use of a laser beam is disclosed in "The Review of Scientific Instruments", Vol. 39, No. 12, page 1916, or Vol. 44, No. 9, page 1193. In this system, a laser beam of a high intensity can be utilized even in a comparatively small-sized laser device. The relation between the laser oscillator and sample air stream detecting space in this system is as illustrated in FIGS. 1(a) and 1(b). Referring to the drawings, reference numeral 1 denotes a laser discharge tube, 2 a laser beam in the resonator, 3 a sample air stream ejected across the laser beam, and 4 the scattered signal light from the small particles contained in the air stream. In the system of FIG. 1(a), a spherical mirror 5 is disposed as a mirror for the laser resonator at one end thereof, a planar mirror 6 in the measurement space at the other end thereof, so as to reduce the diameter of the beam. In the system of FIG. 1(b), planar mirrors or spherical mirros of a comparatively large radius of curvature are used as reflecting mirrors 7, 8 at both sides, and the diameter of a beam in a resonator is nearly constant. In both of these systems, the diameter of the laser beam in the detecting space is large, and the laser beam is not utilized effectively with respect to the energy density thereof. Since the diameter of the laser beam in the detecting space is large, the light-scattering space becomes large, so that the quantity of the Rayleigh scattered light from the molecules of gases, such as N.sub.2 and O.sub.2 in this space increases. Hence, a satisfactory signal-noise ratio cannot be obtained during the measurement of small particles of a low density and a diameter of not more than about 0.1 .mu.m. In the system of FIG. 1(a), the diameter of the beam can be set minimal on the surface of the mirror 6 but it is impossible in practice to provide a detecting space immediately before the mirror 6. Using the system with a laser beam focused on the mirror surface is liable to cause the mirror surface to be deteriorated. The mirror surface is influenced easily by the local soiling thereof and the intensity of the laser beam applied to such a mirror surface is apt to fluctuate.